Capacitive pressure and inertial sensors by Epi-SOI surface micromachining

The benefits of using SOI (silicon on insulator) substrates for MEMS is now well known by industry; single crystalline silicon active layer leading to low stressed structures resistant to mechanical fatigue, low production cost for high volume driven by microelectronics increasing use. In addition, the growth of single crystal silicon upper layer by epitaxy (Epi-SOI) combined with surface micromachining technology allows very interesting designs for miniature high performance sensors. Thus, the typical thickness dimensions of epitaxial layer may vary practically from 5 microns up to 80 depending on design. Firstly, surface micromachining, in particular by deep dry etching, leads to high aspect ratio thick structures with tight tolerances in the micron scale. In addition, wet etching of sacrificial layer oxide permits the controlled release of 3D movable structures (membrane, beam, seismic mass) and direct interconnection to the bulk. As a result, essential features can be achieved for sensors, such as high signal to noise ratio, sharp sensitivity and design flexibility for coping with different measurement ranges (e.g. thickness change of membrane for pressure sensor).